I have never opened that Mac box yet, I think 11 in similar to an ipad, it might be small for the screen.

the specs

1366 by 768 (native), 1344 by 756, and 1280 by 720 pixels at 16:9 aspect ratio
1152 by 720 and 1024 by 640 pixels at 16:10 aspect ratio
1024 by 768 and 800 by 600 pixels at 4:3 aspect ratio

I am trying to figure out to hook up with a 21 inch monitor and an external hard drive (a keyboard too if it's possible), so it should be no difference as a bigger one as long as it's basic system configure meet the requirement of objective-c and xcode's requirement.

I'm developing on a 13 inch air right now and the system is more than capable for any of the development I've thrown at it. The screen could be the only issue if you're used to bigger screens, but full-screen apps really helps a lot imo

I have used an Air for about a year and a half, but just got one of the fully-loaded retina MacBook Pro's.Aside from the small screen (13-inch), I had no complaints at all with using the AirBook - add an external monitor and it's a great set up.

I used a MacBook Pro 15-inch before that and now that I am back to that platform, I must admit I like it best.

I use a 2011 MacBook Air 13" as my primary (actually, the only) development machine, and I'm very happy with it. I don't know if the older model you describe would be as good. I think that the SSD makes a huge difference for development (I/O is often a bottleneck when building code on a system with a regular hard drive).

Try investing in an external monitor, from my experiences, it's a pain using just the MBA when having IB and your implementation files open at the same time (i.e Assistant Editor), trust me, as you get into way bigger projects, it will hurt you.

Seriously considering getting a large 22"+ external monitor!! I use the MBA when I am on the road. The MBA is powerful enough, but the 13" monitor just won't do once you start to use Xcode, especially when you need to use the Interface builder to create UI.

Yup, 1440x900 is not enough for Xcode. I prefer an external screen with a good resolution for Xcode work. (27" with 2560x1440).

On topic: MBA can absolutely deal with most development tasks. I only switched to a MBP because compiling a huge C++ code base would take too long. But if you're using Swift/Obj-C mostly that's not an issue.